The invention relates to a monolithic multi-layer actuator.
With piezoceramic materials use is made of the effect that they are charged when subject to a mechanical load or pulling force and, on the other hand, in the event of electrical charging, they expand or contract. In order to reinforce this effect, monolithic multi-layer actuators are used which consist of a sintered stack of thin films of piezoceramic material (for example lead zirconate titanate) with inserted metal internal electrodes. The internal electrodes lead out of the stack in alternate directions and are connected electrically in parallel by way of external electrodes. For this purpose a basic metallisation is applied to the contact sides of the stack, which metal is connected to the individual internal electrodes. The basic metallisation is reinforced by means of areal or partial covering of the basic metallisation with solder. This reinforcement produces, on the one hand, the necessary material cross section in order to carry the high currents which occur during operation of the actuator (about 20-80 amperes). On the other hand, the soldering-on of electrical supply leads is made possible.
If an electric voltage is applied to the external electrodes, the piezofilms expand in the field direction. The nominal expansion of all the piezoceramic material is achieved already at low electrical voltages by means of the mechanical series connection of the individual piezofilms.
The indicated monolithic multi-layer actuators are described in detail in DE 40 36 287 C2. Use in a flow throughput control valve is also indicated here.
Piezoceramic materials are naturally brittle and have only a low tensile strength (about 80.10.sup.6 Pa). This is further reduced with multi-layer actuators by way of the laminar arrangement of the internal electrodes and the anisotropy of the strength which occurs during polarization. The maximum permitted tensile stress is often exceeded already during polarization with the result that cracking inevitably occurs.
However, there is no indication that this type of cracking leads to failure of the actuators under normal operating conditions.
The growth of cracking within the ceramic materials can additionally be influenced considerably by grain size, grain boundary composition and porosity. Under suitably set conditions cracks do not run in a transcrystalline manner and are rapidly stopped by energy sinks at grain boundaries and pores. Already after about 1000 loading cycles the crack growth has largely stopped and even after long operating times (10.sup.9 cycles) only increases slightly.
However, with high dynamic loads of the multi-layer actuators, these cracks can become critical if the cracks in the ceramic material cut through the basic metallisation and the applied layer of solder. In the most favourable case only individual piezofilms are then separated. However, voltage flashovers at the crack edge occur substantially more frequently, which voltage flashovers lead to a destruction of the multi-layer actuator because the entire operating current flowing at this point is disconnected.